1. FIELD OF THE INVENTION
The present invention relates to a rear projection screen, more particularly, to a rear projection screen satisfactory in both optical properties and mechanical properties.
2. DESCRIPTION OF THE PRIOR ART
In general, a rear projection screen comprises a light diffusion layer and a transparent support, although rear projection screens including both these elements and rear projection screens consisting of a light diffusion layer alone are known. At present, the latter type of rear projection screens are most commonly used.
The term "light diffusion layer" designates a layer having the action of diffusing light. For instance, (a) a diffusion layer having a surface in the form of a mat such as ground glass (hereinafter referred to as a "mat type diffusion layer"), (2) a diffusion layer prepared by dispersing light scattering particles in a transparent binder (hereinafter referred to as a "dispersion type diffusion layer"), (3) a diffusion layer prepared by coating an organic binder with light scattering particles dispersed therein onto a transparent sheet such as a glass plate (hereinafter referred to as a "coating type diffusion layer"), (4) a diffusion layer comprising a wax sheet, (5) a diffusion layer comprising a polymeric material sheet, etc., are known.
The term "transparent support" designates a layer serving to improve the mechanical properties and the optical properties of a diffusion layer. For instances, (a) a transparent plate such as a glass plate, an acrylic resin plate, etc., (b) a surface optical element plate having on a transparent sheet a surface microoptical element structure in the form of a Fresnel lens, lenticular lens, V-groove, or the like, (c) a transparent plastic sheet having a venetian blind-like louver structure (hereinafter merely referred to as an "LCF"), etc., are known.
At present, as described above, rear projection screens consisting of a light diffusion layer alone are often commonly used. However, this type of rear projection screen is not always satisfactory in both optical and mechanical properties.
As is described in detail in U.S. patent application Ser. No. 654,830 filed on Feb. 3, 1976, although a mat type diffusion layer, a dispersion type diffusion layer (for example, as described in U.S. Pat. No. 2,180,113), a coating type diffusion layer, etc., are fairly satisfactory in mechanical properties, their optical properties such as scintillation, image reproduction range, light redistribution capability, resolving power, etc., are not satisfactory.
On the other hand, in the case of a diffusion layer comprising a wax sheet, the optical properties thereof are excellent, but the mechanical properties thereof are markedly inferior. Thus, it has been difficult to use a wax sheet alone as a light diffusion layer.
Although a polymer light diffusion layer is slightly inferior to a wax light diffusion layer in optical properties, the polymer light diffusion layer has the advantages over the wax light diffusion layer in that its mechanical strength is excellent, its surface hardness is high, and workability is good.
Hence, attempts have been made to produce rear projection screens not only from a light diffusion layer comprising wax alone but to use such a light diffusion layer comprising wax and a transparent support such as a glass plate, etc., together. However, since wax generally has low affinity to a transparent support such as a glass plate, etc., even though if a layer of wax is provided on the glass plate by coating as in a coating type diffusion layer, the wax layer peels or cracks with ease. Thus, in general, rear projection screens having a sandwich structure in which a wax diffusion layer is sandwiched between two transparent supports such as glass plates, etc., have been used.
With a sandwich structure, however, the affinity of a wax diffusion layer to a transparent support such as a glass plate, etc., is not improved, and thus the wax layer peels or air bubbles enter into the interface with the passage of time, resulting in insufficient durability of the rear projection screen. Moreover, since such a sandwich structure has interfaces of different refractive indexes (different interfaces) on both sides of the light diffusion layer, light is refracted and reflected, resulting in a loss of light, and thus the excellent optical properties of the wax are undesirably deteriorated.
Japanese Utility Model No. 21,110/1969 discloses a rear projection screen wherein a wax light diffusion layer is sandwiched between transparent members such as glass plates, etc., and Japanese Utility Model No. 14,236,1968 discloses a rear projection screen wherein a polymer light diffusion layer is sandwiched between transparent supports, such as glass plates.
with such rear projection screens, however, since their light redistribution capability is limited by the diffusion capability of the wax or polymer light diffusion layer, it has been quite difficult to redistribute the light passed through the rear projection screen to the desired range.
That is to say, it is, in general, not necessary that the light redistribution capability of a rear projection screen be equal in all directions, and it is often desired to redistribute light to a certain specific range from the viewpoint of efficiently utilizing the light passed through the light diffusion layer. For instance, where a plurality of viewers view an image projected onto the rear projection screen at the same time, it is important that the diffusion capability in the horizontal direction be larger than that in the vertical direction, whereas when a single viewer views an image on the rear projection screen from fixed position, it is preferred that transmitted light be focussed only at the observation range of the viewer, since the transmitted light can be efficiently utilized. In rear projection screens with the above sandwich structure, however, the light redistribution capability is limited by the diffusion capability of the wax diffusion layer, thereby making it quite difficult to meet the above requirements.
Moreover, a desired rear projection screen is required to faithfully reproduce the image of an original microfilm, that is, pattern, color tone, and contrast. Irrespective of these requirements, however, no consideration has hitherto been paid to these points by the art. Hence the art has never chosen color tone so as to faithfully reproduce color images, although the color tone has been chosen to harmonize with that in the room where viewing is conducted or to reduce eye fatigue.
Further, while a light diffusion layer comprising a polymeric material sheet is fairly satisfactory in optical properties (next to wax), its mechanical strength is still insufficient, and, as in the case of wax, a light diffusion layer comprising a polymeric material sheet has generally been sandwiched between transparent supports such as glass plates, as described in, for example, Japanese Utility Model Publication No. 14,236/1968.
Problems also occur in this case in that the above optical defects inherent to a sandwich structure are inevitably caused and in that air bubbles enter into the interface with the passage of time due to the insufficient affinity of the polymeric material sheet to the glass plates, and thus it has been quite difficult to provide such rear projection screens satisfactory in both optical properties and in mechanical properties.
It could be considered, therefore, to produce a light diffusion layer, particularly a wax diffusion layer or polymeric material sheet, and a transparent support from materials having affinity to each other in order to bring both into direct contact without employing a sandwich structure as is shown in FIG. 1, thereby removing the above described defects. It has been, however, quite difficult and, for practical purposes, impossible, to select the materials of the light diffusion layer and the transparent support in such a way that they have good affinity for each other. This is because the materials of the light diffusion layer and the transparent support are principally selected taking into consideration the optical properties of both, as a result of which their affinity is given no or almost no consideration.
Further, in Japanese Utility Model Publication No. 14,236/1968, since the thickness of the transparent support used is high, stiffness is high, (though not disclosed, typically 2 to 5 mm would be conventional) if the difference in the coefficient of thermal expansion between polymer diffusion layer and the transparent support is high, stress due to the difference is coefficient of thermal expansion results at the interface between the polymer diffusion layer and the transparent support due to the heat applied during the use thereof. Thus, problems can arise in that shearing occurs at the interface therebetween and the rear projection screen itself can be broken.
Similar disclosure occurs in Japanese Utility Model No. 21,110/1969, i.e., a wax diffusion layer is bonded between transparent supports, and the transparent supports utilized have a thickness on the order of 2 to 5 mm, i.e., stress is generated due to the difference in coefficient of thermal expansion between the wax diffusion layer and the transparent support due to the heat applied during the use thereof.
In all of the above systems wherein stress generation is a problem, this is primarily due to the fact that diffusion layers and transparent supports have been selected primarily from the viewpoint of their optical properties, with little attention being given to minimizing any difference in coefficient of thermal expansion therebetween.
Moreover, when the light diffusion layer and the transparent layer are bonded to each other using an intermediate layer, it is a difficult problem to select a material for the intermediate layer which permits the most effective functioning of the rear projection screen without deteriorating the functions thereof.
In brief, as described above, it has been quite difficult to provide rear projection screens satisfactory in both mechanical and optical properties by bonding a light diffusion layer and a transparent support directly.